Faster transport for in-process communications

[akiradeveloper]

In certain application, sending requests from RaftApp to RaftCore happens. For example, if it's a distributed storage application, it may want to send new writes to rebuild a missing part of the data.

In the current lol 0.7.0, it is possible to do this by sending requests through gRPC as usual client but it is not efficient as it goes through network stack.

But ideally, transport can be a simple channel.

My idea here is to expose such server from RaftCore as well as tonic::Server.

My first choice is tarpc which is a non-official Google product but well-maintained. It seems to be transport-agnostic

Pluggable transport: any type impling Stream<Item = Request> + Sink can be used as a transport to connect the client and server.

and it supports both channel and tcp.

[akiradeveloper]

Tarpc doesn't support streaming (google/tarpc#189) which means we can't entirely replace tonic with tarpc because we need streaming in log replication.

[akiradeveloper]

deprecated